The present invention relates to a method of reducing the nitrogen oxide level in the flue gases issuing from combustion units and, more particularly, to the reduction of NO.sub.x levels by introduction of reducing agents into contact with gases containing nitrogen oxides, and to the finalization of the reduction by bringing the gas into catalytic reduction. The present invention also relates to a steam generation boiler with improved nitrogen reduction facilities.
The invention relates to method of lowering the nitrogen oxides content of the flue gases emanating from the reactions of substantially any combustible fuel, including solid fuels, sludges, gaseous fuels, or the like. In particular, the invention provides an improved fluidized bed combustion process, in which the effluent stack gases can be managed economically to meet the current environmental standards.
Reduction of nitrogen oxides emissions from the exhaust gases or flue gases before they are released into the atmosphere has been a prolific topic of discussion in the field of environmental aspects of energy production by combustion of fuel material. Because NO.sub.x emissions are related to various environmental problems, the minimization of NO.sub.x release from combustion systems is an ongoing concern.
It is evident that nitrogen oxide emissions result from any combustion reaction where air is present and/or the fuel used contains nitrogen. Fluidized bed combustion of fuel is a well known practice and found to be beneficial in reducing nitrogen oxide emissions due to its relatively low operating temperature. In fluidized bed combustion, air is typically introduced through a plenum where it is distributed through an air distribution grid. Fuel, fluidizing solids, and possibly sorbents such as limestone or dolomite, are fluidized and they react in the furnace at temperatures normally in the range of about 700-1200.degree. C. Nitrogen oxides are generated in burning of any fuel as a result of thermal fixation of nitrogen in the air and the conversion of fuel nitrogen. The former reaction is favored at high temperatures (above about 950.degree. C.) while the latter is of greater concern at lower temperatures, e.g., those generally found in fluidized bed combustion systems.
U.S. Pat. No. 3,900,554 suggests non-catalytic removal of nitrogen oxides from the flue gases which have exited a conventional furnace, by injecting ammonia (NH.sub.3) into the effluent stream while it is at a temperature of about 871-1093.degree. C. (i.e. about 1600-2000.degree. F.). European patent publication 176,293 also discloses use of NH.sub.3 for NO.sub.x control via ammonia injection into a flue gas stream prior to its entry into a centrifugal separator. Many other patents have suggested use of ammonia with a catalyst, so that the ammonia is injected into the gases prior to catalytic reduction. U.S. Pat. No. 4,393,031 suggests injection of ammonia into the gases and, after mixing the gas with ammonia, passing of the mixture through a catalytic reactor.
The methods suggested by the prior art are advantageous but still may have several shortcomings. Non-catalytic reduction of NO.sub.x by injecting ammonia into flue gases has a limited capacity to reduce nitrogen oxide emissions, while the molar ratio of NH.sub.3 /NO.sub.x may increase to such a high level that "NH.sub.3 slip" will emerge. This causes undesirable ammonia emissions with the flue gases into the atmosphere, as well as possible binding of ammonia in ashes. The suggested method of injecting ammonia into the gas prior to its contact with catalyst has better reducing capabilities. However, catalytic nitrogen oxide reduction requires a large amount of catalyst. As a result, high space-consuming vessels are needed to support the catalyst layers. In commercial scale plants, this kind of catalyst vessel may normally be even higher than 7-10 m. Substantial pressure losses will also take place in this kind of reduction systems.
According to the present invention a method of reducing nitrogen oxide emissions into the atmosphere from a combustion process is provided in which efficient reduction is achieved and the shortcomings of the prior art methods discussed above are overcome. According to the present invention a method of reducing NO.sub.x emissions from a combustion steam generation process is provided in which efficient reduction is achieved with a compact size steam generator. According to the present invention a method of reducing NO.sub.x emissions from a combustion process by efficient NO.sub.x reduction in catalytic treatment is provided in which the pressure losses are low, typically less than about 400 Pa.
The present invention also comprises a fluidized bed steam generation boiler system with better reduction facilities for NO.sub.x emissions than in the prior art, and in which efficient reduction is achieved with a compact size fluidized bed reactor.
According to one exemplary method of the present invention a method of purifying combustion gases from a steam generation boiler system, which comprises a fuel reaction chamber and a flue gas convection section operatively connected to the reaction chamber, having heat transfer elements for extracting heat from the gases, is provided. The method comprises the steps of: (a) maintaining combustion reactions in the reaction chamber resulting in the production of hot gases containing nitrogen oxides; (b) discharging the hot gases from the reaction chamber and leading them into the convection section; (c) cooling the gases in the flue gas convection section; (d) reducing nitrogen oxides in a first reducing stage by bringing the hot gases into contact with a reducing agent; and then (e), reducing nitrogen oxides in a second reducing stage by subjecting the gases containing the reducing agent of the first reducing stage to a catalytic NO.sub.x reduction in the flue gas convection section.
The invention also relates to a steam generation boiler system which includes the following elements: A fuel reaction chamber and a flue gas convection section operatively connected to the reaction chamber. The convection section has heat transfer elements for extracting heat from flue gases. Means are provided for introducing reducing agent into the flue gases in the reaction chamber; and catalytic nitrogen oxide reducing means are provided in the convection section on the opposite side of the introducing means from the reaction chamber.
The invention also relates to a method of purifying combustion gases from a fluidized bed steam generation plant which includes a fluidized bed reaction chamber, a particle separator connected to the reaction chamber, and a flue gas convection section connected to the particle separator and having heat transfer elements for extracting heat from the gases. This method comprises the steps of: (a) maintaining combustion reactions in a fluidized bed of solids in the fluidized bed reaction chamber, resulting in the production of hot flue gases; (b) discharging the hot gases and particles entrained therewith from the reaction chamber and leading the gases and particles into the particle separator; (c) separating particles from the gases in the separator; (d) in a first reducing stage, bringing the hot gases into contact with a reducing agent which effects reduction of the NO.sub.x content of the gases under non-catalytic conditions; (e), conveying the flue gases into the flue gas convection section, and cooling the gases therein; and, (f) in a second reducing stage, subjecting the gases containing the reducing agent of the first reducing stage to a catalytic NO.sub.x reduction in the flue gas convection section after the practice of step (e).
The amount of nitrogen oxides in the hot gases is reduced according to the present invention in a combination of two stages disposed in series, while producing steam in a steam generation boiler system, thus resulting in gases essentially free of nitrogen oxides and eliminating the possibility of NH.sub.3 (or like reducing agent) slip in the exhausted flue gases. The present invention utilizes heat transfer surfaces in a convection section to establish stabilized temperature conditions for catalytic conversion.
According to the present invention, NO.sub.x reducing agent, preferably ammonia, is injected into the hot combustion gases in the reaction chamber, and/or in a section connecting the reaction chamber and the convection section, at a temperature &gt;800.degree. C. This effects non-catalytic reduction of nitrogen oxides in the hot gases. Injection into the locations described above causes no additional pressure losses. According to the invention, it is preferable to adapt the injection location(s) to the steam generation load of the plant, thus ensuring that an optimum injection temperature and retention time of ammonia are maintained in the first stage of reduction, in all operating conditions of the fluidized bed steam generation boiler system.
The gases--which still contain nitrogen oxides and ammonia--are caused to pass the heat transfer surfaces in the convection section of the steam generation plant, thereby lowering the temperature of the gases. After having been cooled to a temperature of about 300-500.degree. C., the gases are introduced into the second reduction stage for catalytic reduction of nitrogen oxides. In the second stage the ammonia previously injected into the hot gases is present, and essentially no additional reducing agent is needed in normal operation. The temperature is selected according to the requirements of the catalyst used and, once selected, the temperature must be maintained stable within certain limits for ensuring that reduction takes place. According to the invention, it is preferable to cool the gases to the selected temperature, which is typically in the range of about 300-500.degree. C., by appropriate heat transfers in the convection section before the catalytic treatment. In this manner the temperature of catalytic treatment may be stabilized, and also preferably maintained in the second stage within the range of about .+-.25.degree. C. of the optimum working temperature of the catalyst used. The stable temperature conditions are readily maintained by regulating the flow rate of the heat transfer medium at least in a heat exchanger preceding the catalyst in the convection section. This is preferably performed according to the load of the steam generator, thereby always obtaining an optimum working temperature of the catalyst, with varying loads of the plant.
Since the major portion of reduction has taken place in the first reduction stage, the catalyst in the second stage is preferably disposed in connection with the convection section, most preferably inside the convection section after the heat transfer surfaces. The dimension of the catalyst layer according to the present invention is so small that it may be disposed in an appropriate location in the convection section, so that the temperature of the catalyst may be maintained at its optimum operating level. It has been found that the required reduction of nitrogen oxides results if the gases are disposed to flow through the catalyst over a linear flow distance of less than about 2 m (e.g. less than 1 m) in a commercial size plant. Thus, according to the present invention, much less catalytic surface is required than in the prior art methods. Consequently, pressure losses are smaller. The catalyst required by the present invention results in pressure losses which are at least 50% less than in prior art methods (e.g. less than about a 400 Pa pressure reduction). This results in considerable savings in operating costs, while still providing adequate NO.sub.x reduction without excessive ammonia slip.
Under some operating conditions, such as a low load, there may be an excess of injected ammonia after the first reduction stage. In such cases the second reduction stage, according to the invention, while reducing the nitrogen oxide emissions, simultaneously removes the gaseous NH.sub.3 slip from the gases. This makes the present invention with two reducing stages even more attractive. It is possible to inject such amounts of ammonia that the reduction is at its maximum without a risk of a harmful NH.sub.3 slip and its entrainment into atmosphere with the exhausted flue gases.
It should be understood that any known nitrogen reducing agent may be utilized in connection with the present invention, but preferably the reducing agent is selected from the group essentially consisting of amine-containing agent, ammonia or urea, or ammonia producing precursor.